Monitoring of compound resting membrane potentials of cell cultures with ratiometric genetically encoded voltage indicators

An Ultrasensitive Genetically Encoded Voltage Indicator Uncovers the Electrical Activity of Non-Excitable Cells

Most animal cell types are classified as non-excitable because they do not generate action potentials observed in excitable cells, such as neurons and muscle cells. Thus, resolving voltage signals in non-excitable cells demands sensors with exceptionally high voltage sensitivity. In this study, the ultrabright, ultrasensitive, and calibratable genetically encoded voltage sensor rEstus is developed using structure-guided engineering. rEstus is most sensitive in the resting voltage range of non-excitable cells and offers a 3.6-fold improvement in brightness change for fast voltage spikes over its precursor ASAP3. Using rEstus, it is uncovered that the membrane voltage in several non-excitable cell lines (A375, HEK293T, MCF7) undergoes spontaneous endogenous alterations on a second to millisecond timescale. Correlation analysis of these optically recorded voltage alterations provides a direct, real-time readout of electrical cell-cell coupling, showing that visually connected A375 and HEK293T cells are also largely electrically connected, while MCF7 cells are only weakly coupled. The presented work provides enhanced tools and methods for non-invasive voltage imaging in living cells and demonstrates that spontaneous endogenous membrane voltage alterations are not limited to excitable cells but also occur in a variety of non-excitable cell types.

Merging BioActuation and BioCapacitive properties: A 3D bioprinted devices to self-stimulate using self-stored energy

Biofabrication of three-dimensional (3D) cultures through the 3D Bioprinting technique opens new perspectives and applications of cell-laden hydrogels. However, to continue with the progress, new BioInks with specific properties must be carefully designed. In this study, we report the synthesis and 3D Bioprinting of an electroconductive BioInk made of gelatin/fibrinogen hydrogel, C2C12 mouse myoblast and 5% w/w of conductive poly (3,4-ethylenedioxythiophene) nanoparticles (PEDOT NPs). The influence of PEDOT NPs, incorporated in the cell-laden BioInk, not only showed a positive effect in cells viability, differentiation and myotube functionalities, also allowed the printed constructs to behaved as BioCapacitors. Such devices were able to electrochemically store a significant amount of energy (0.5 mF/cm2), enough to self-stimulate as BioActuator, with typical contractions ranging from 27 to 38 μN, during nearly 50 min. The biofabrication of 3D constructs with the proposed electroconductive BioInk could lead to new devices for tissue engineering, biohybrid robotics or bioelectronics.

Fluorescence Imaging of Cell Membrane Potential: From Relative Changes to Absolute Values

Membrane potential is a fundamental property of biological cells. Changes in membrane potential characterize a vast number of vital biological processes, such as the activity of neurons and cardiomyocytes, tumorogenesis, cell-cycle progression, etc. A common strategy to record membrane potential changes that occur in the process of interest is to utilize organic dyes or genetically-encoded voltage indicators with voltage-dependent fluorescence. Sensors are introduced into target cells, and alterations of fluorescence intensity are recorded with optical methods. Techniques that allow recording relative changes of membrane potential and do not take into account fluorescence alterations due to factors other than membrane voltage are already widely used in modern biological and biomedical studies. Such techniques have been reviewed previously in many works. However, in order to investigate a number of processes, especially long-term processes, the measured signal must be corrected to exclude the contribution from voltage-independent factors or even absolute values of cell membrane potential have to be evaluated. Techniques that enable such measurements are the subject of this review.